Level-3.1: Storage correction
The storage term is added to the CO2 flux of the enclosed-path LI-7200 (IRGA72): NEE_L3.1 is FC plus the single-point storage term SC_SINGLE from EddyPro. The storage term used is in SC_SINGLE_gfRMED_L3.1, and every record with a measured FC has one.
LE and H get no storage term, as in Pastorello et al. (2020). Their Level-3.1 columns LE_L3.1 and H_L3.1 keep the level in the name but equal LE and H.
Method, evidence and checks:
01_FPPC_IRGA72_NEE, Step 3: the screening and its check plot.- diive: storage term: how the screening works.
Screening
The single-point storage term is the change in the CO2 concentration at 47 m between two consecutive records, so errors in the concentration enter it directly. Two steps screen it before it is added; the values they remove are filled with a rolling median.
- Next to gaps in the concentration. After a signal loss the LI-7200 can restart with readings far from the real concentration and return to it over hours, for example 695 ppm on 21 December 2017. The return entered the storage term as a large negative flux, while the jump at the restart fell into the gap and was never counted. Over 2016 to 2026 these one-sided changes summed to about −150 gC m-2. Next to each gap in
CO2_MOLAR_DENSITY, the storage values are discarded until the concentration is back within 3 robust standard deviations of its median at the same time of day (±7 days). - Spikes. Isolated spikes, found with a Hampel filter on the double differences, are replaced together with one record on either side by the mean of that block. The block keeps its sum, so the storage term stays balanced.
After the screening the cumulative storage term at measured fluxes stays close to zero, except in December 2017 to January 2018 and in November 2020, when the analyser was failing. The fluxes of these periods are rejected by the signal strength test or removed by hand at Level 3.2, so their storage term does not enter the flux. The numbers of each run are in the notebook.
Columns
| Column | Content |
|---|---|
NEE_L3.1 |
FC plus the storage term, µmol m-2 s-1 |
SC_SINGLE_gfRMED_L3.1 |
Storage term used, µmol m-2 s-1 |
FLAG_SC_SINGLE_gfRMED_L3.1_ISFILLED |
0 measured, 1 filled because missing, 2 spike block replaced by its mean, 3 discarded next to a gap and filled |